A honeycomb structure is usually a ceramic product whose outer shape is a columnar shape or a prism shape. This honeycomb structure includes a plurality of cells defined by partition walls being made of a ceramic porous material and having a large number of pores, whose shape looks like a honeycomb nest. Such a honeycomb structure including the cells which become through channels of a gas is often used as a filter or a catalyst carrier.
The honeycomb structure can be manufactured through extrusion of a forming raw material by use of an extrusion forming machine to which a honeycomb structure forming die for exclusive use (also simply referred to as die) is attached. This die is obtained by forming, in a die base body made of a pure metal, an alloy or the like, back holes (often referred to as the introduction holes) through which the forming raw material (often referred to as the kneaded clay) is introduced, and slits which communicate with the back holes. The slits have a shape matching a sectional shape of cells, and has a width corresponding to a thickness of each partition wall of the honeycomb structure. Each of the back holes has a diameter larger than a width of each slit, and the back holes are provided at positions corresponding to intersections of the slits. According to the extrusion forming by use of such a die, the forming raw material introduced through the back holes moves toward the slits each having a small width, is extruded through the slits, and is discharged as a honeycomb-structure formed body (often referred to as the honeycomb formed body). Then, the honeycomb formed body is fired, whereby the honeycomb structure can be obtained.
Such a die can be obtained by forming the above slits and back holes in the die base body which is not provided with any slit or back hole. Examples of slit forming means include electric discharge machining (EDM) using an electrode, cutting, grinding, and electrolytic machining. For example, Patent Documents 1 and 2 disclose means for manufacturing a die which forms a honeycomb structure including cells having a hexagonal sectional shape. In Patent Documents 1 and 2, a combtooth-like electrode including a large number of thin flat plate-like projections adjusted to the slits is used. Moreover, the slits are processed by discharging electricity from the thin flat plate-like projections.